1. Field of the Invention
The present invention relates to asphalt roofing shingles, protective granules for such shingles, and processes for making such granules and shingles.
2. Brief Description of the Prior Art
Pigment-coated mineral rocks are commonly used as color granules in roofing applications to provide aesthetic as well as protective functions to the asphalt shingles. Roofing granules typically include a core formed by crushed and screened mineral materials, which are subsequently coated with one or more color coating layers comprising a binder in which is dispersed one or more coloring pigments, such as suitable metal oxides. Inorganic binders are typically employed. The binder can be a soluble alkaline silicate that is subsequently insolubilized by heat or by chemical reaction, such as by reaction between an acidic material and the alkaline silicate, resulting in an insoluble colored coating on the mineral particles. The coating layer may also include additives for long term outdoor durability and functionality.
When an alkali metal-silicate binder such as sodium silicate is employed in the preparation of algae-resistant granules, the binder can include a heat-reactive aluminosilicate material, such as clay, for example, kaolin clay. Alternatively, it is possible to insolubilize the metal silicate binder chemically by reaction with an acidic material, for example, ammonium chloride, aluminum chloride, hydrochloric acid, calcium chloride, aluminum sulfate, and magnesium chloride, such as disclosed in U.S. Pat. Nos. 2,591,149, 2,614,051, 2,898,232 and 2,981,636, or other acidic material such as aluminum fluoride. The binder can also be a controlled release sparingly water soluble glass such as a phosphorous pentoxide glass modified with calcium fluoride, such as disclosed in U.S. Pat. No. 6,143,318. The most commonly used binder is a mixture of an alkali metal silicate and an alumino-silicate clay material.
Although inexpensive, coating binders prepared from mixtures of an alkali metal silicate and an alumino-clay material have drawbacks. In particular, the pot life of coating compositions including such binders depends significantly on the humidity and ambient temperature.
Dark blotches or streaks sometimes appear on the surfaces of asphalt shingles, especially in warmer humid climates, because of the growth of algae and other microorganisms. The predominant species responsible is Gloeocapsa sp, a blue-green algae. Other microbial growth, including fungi, moss and lichen, can also occur under proper conditions, for example, in a shady and/or persistently damp environment. In addition to being aesthetically unpleasant, the discoloration can lead to heat buildup and accelerate premature roofing failure. Eventually, severe discoloration of the entire roof can occur.
Various methods have been used in an attempt to remedy the roofing discoloration. Washing the roof surfaces with dilute cleaning solutions containing a strong oxidizer such as bleach can remove the algae from roofs. However, frequent washing and cleaning with cleaning solutions is required, since the effective duration of such treatments is rather short. In addition, topical treatments with organic algaecides have been used. However, such topical treatments are also usually effective only for short term, typically one to two years.
If the freshly cleaned surfaces are treated with a coating containing some form of biocides, the antimicrobial properties could remain for a longer period of time, between five to seven years. To prevent algal growth, various types of biocides have been used. The most commonly used biocides are metals and inorganic metal oxides, such as, for example zinc metal granules and copper oxide-coated granules. However, these biocides typically persist for around ten years, and in some limited cases, for periods approaching fifteen years. One drawback is these compounds are effective against only one microbe, Gloeocapsa sp. At the same time, the service life of roofing products can extend considerably longer than ten to fifteen years, depending on the composition and structure of the roofing materials employed to construct the roof.
Increased public awareness of algae infestation on roofing shingles has led to higher demand for products that provide algae resistance. Algae infestation on roofing shingles can cause severe discoloration to the roof and is therefore highly undesirable to homeowners. In addition, the discoloration leads to darkened color on the roof and hence can have undesirable effects for increasing solar heat absorption. Some roofing manufacturers would carry only algae resistant (AR) shingles in their product lines, and even for regions that were once considered non-algae prone such as the Northeast and the Great Lakes areas. Furthermore, environmental friendly products that would not pollute the ecosystem are also more attractive.
Companies, including Minnesota Mining and Manufacturing (3M Company) and ISP Mineral Products Inc., have commercialized several algaecidal granules that are effective in inhibiting algae growth.
A common method used to prepare algae-resistant roofing granules generally involves two major steps. In the first step, metal oxides such as cuprous oxide and/or zinc oxide are added to a clay and alkali metal silicate mixture. The mixture in turn is used to coat crushed mineral rocks. The mixture is rendered insoluble on the rock surfaces by firing at high temperatures, such as about 500 degrees C., to provide a ceramic coating. In the second step, the oxide covered rocks are coated with various color pigments to form colored algae-resistant roofing granules. The algae-resistant granules, alone, or in a mixture with conventional granules, are then used in the manufacture of asphalt shingles using conventional techniques. The presence of the algae-resistant granules confers algae-resistance on the shingles.
Algae resistant shingles are disclosed, for example, in U.S. Pat. No. 5,356,664 assigned to Minnesota Mining and Manufacturing Co., which discloses the use of a blend of algae-resistant granules and non-algae-resistant granules. The algae-resistant granules have an inner ceramic coating comprising cuprous oxide and an outer seal coating initially devoid of copper. U.S. Pat. No. 3,507,676 discloses roofing granules containing zinc, zinc oxide, or zinc sulfide, as an algaecide and fungicide.
Roofing granules containing cuprous oxide have been used as algae resistant (AR) granules since the mid-1990's. Photocatalytic coatings containing anatase nano-titanium oxide have been reported for construction materials and sanitary ware in different applications including self-cleaning, anti-fogging, and anti-bacterial material, such as disclosed in U.S. Pat. No. 6,294,247, U.S. Patent Publication 2005-0277543 A1, and U.S. Patent Publication 2006-0014050 A1. Coating compositions consisting of an alkali metal silicate binder and photocatalytic particles have been described as possessing anti-algal properties in building materials, as disclosed in U.S. Pat. No. 6,569,520 and U.S. Pat. No. 6,881,701. Phosphate-bonded ceramic binders have been developed for encasing hazardous wastes, such as disclosed in U.S. Pat. No. 6,204,214. Also, field-applied clear coatings containing biocides have been used to prevent algae infestation on shingles.
There is a need for a curable binder composition for roofing granules that permits a flexible adjustment of cure rate and cure time. In addition, there is a need for a binder composition that can be formulated to meet different manufacturing conditions. There is also a need for a binder with reduced dependence on the temperature and humidity of the manufacturing environment.
In addition, there is a continuing need for algae-resistant roofing products that that do not leach out metal species (such as copper or zinc ions) or organic biocides. At the same time, there is a continuing need for roofing granules that provide effective algaecidal properties and algae resistance to roofing products, preferably over extended periods of time.